Whilst the growing number of identity management systems have the potential to reduce
the threat of identity attacks, major deployment problems remain because of the lack of
interoperability between such systems. In this paper
we propose a novel, simple scheme to provide interoperability between two
of the most widely discussed identity management systems, namely
CardSpace and OpenID\@. In this scheme, CardSpace users are
able to obtain an assertion token from an OpenID-enabled identity
provider, the contents of which can be processed by a CardSpace-enabled
relying party. The scheme, based on a browser extension, is
 transparent to OpenID providers and to the CardSpace identity selector,
and only requires minor changes to the operation of a CardSpace-enabled relying party.
We specify its operation and also describe an implementation of a proof-of-concept prototype.
Additionally, security and operational analyses are provided.